local agent run failing on unknown runtime

**Describe the bug**

Our [buildspecs](https://github.com/aws/s2n-tls/blob/main/codebuild/spec/buildspec_ubuntu_integ_openssl111.yml#L24) uses runtime-version python 3.x, but the local codebuild is erroring out expecting a specific version.

**To Reproduce**
Steps to reproduce the behavior:
- checkout https://github.com/aws/s2n-tls/
- setup local codebuild per the docs
- run `./codebuild_build.sh -i "$IMAGE" -l amazon/aws-codebuild-local:aarch64 -a ./output -m -b codebuild/spec/buildspec_ubuntu_integ_openssl111.yml`

**Expected behavior**

- Runs with a python 3.x environment

**Logs**

```
Attaching to agent-resources_agent_1, agent-resources_build_1
agent_1 | [Container] 2021/10/25 21:22:20 Waiting for agent ping
agent_1 | [Container] 2021/10/25 21:22:22 Waiting for DOWNLOAD_SOURCE
agent_1 | [Container] 2021/10/25 21:22:23 Phase is DOWNLOAD_SOURCE
agent_1 | [Container] 2021/10/25 21:22:23 CODEBUILD_SRC_DIR=/codebuild/output/src917472404/src
agent_1 | [Container] 2021/10/25 21:22:23 YAML location is /codebuild/output/srcDownload/src/codebuild/spec/buildspec_ubuntu_integ_openssl111.yml
agent_1 | [Container] 2021/10/25 21:22:23 Processing environment variables
agent_1 | [Container] 2021/10/25 21:22:23 Phase complete: DOWNLOAD_SOURCE State: FAILED
agent_1 | [Container] 2021/10/25 21:22:23 Phase context status code: YAML_FILE_ERROR Message: Unknown runtime version named '3.x' of python. This build image has the following versions: 3.7, 3.8, 3.9
agent_1 | [Container] 2021/10/25 21:22:23 Runtime error (*clienterr.PhaseContextError: Unknown runtime version named '3.x' of python. This build image has the following versions: 3.7, 3.8, 3.9)
```

**Platform (please complete the following information):**
- Host: AL2, guest: ubuntu18 (standard/4.0)

**Additional context**
Add any other context about the problem here.